Not applicable.
The present invention relates to a method and apparatus for the preparation of hydrocarbons from synthesis gas, i.e., a mixture of carbon monoxide and hydrogen, typically labeled the Fischer-Tropsch process. Particularly, this invention relates to a method and apparatus for in situ water removal in multi-phase column reactors operating at Fischer-Tropsch conditions.
Large quantities of methane, the main component of natural gas, are available in many areas of the world, and natural gas is predicted to outlast oil reserves by a significant margin. However, most natural gas is situated in areas that are geographically remote from population and industrial centers. The costs of compression, transportation, and storage make its use economically unattractive. To improve the economics of natural gas use, much research has focused on the use of methane as a starting material for the production of higher hydrocarbons and hydrocarbon liquids, which are more easily transported and thus more economical. The conversion of methane to hydrocarbons is typically carried out in two steps. In the first step, methane is converted into a mixture of carbon monoxide and hydrogen (i.e., synthesis gas or syngas). In a second step, the syngas is converted into useful hydrocarbons.
This second step, the preparation of hydrocarbons from synthesis gas, is well known in the art and is usually referred to as Fischer-Tropsch synthesis, the Fischer-Tropsch process, or Fischer-Tropsch reaction(s). Fischer-Tropsch synthesis generally entails contacting a stream of synthesis gas with a catalyst under temperature and pressure conditions that allow the synthesis gas to react and form hydrocarbons.
More specifically, the Fischer-Tropsch reaction is the catalytic hydrogenation of carbon monoxide to produce any of a variety of hydrocarbon products ranging from methane to higher alkanes. Research continues on the development of more efficient Fischer-Tropsch catalyst systems and reaction systems that increase the selectivity for high-value hydrocarbons in the Fischer-Tropsch product stream.
There are continuing efforts to design reactors that are more effective at producing these desired products. Product distribution, product selectivity, and reactor productivity depend heavily on the type and structure of the catalyst and on the reactor type and operating conditions. Catalysts for use in such synthesis usually contain a catalytically active metal of Groups 8, 9, or 10 (in the New notation of the periodic table of the elements, which is followed throughout). In particular, iron, cobalt, nickel, and ruthenium have been abundantly used as the catalytically active metals.
Originally, the Fischer-Tropsch synthesis was operated in packed bed reactors. These reactors have several drawbacks, such as temperature control, that can be overcome by gas-agitated slurry reactors or slurry bubble column reactors. Gas-agitated reactors, sometimes called xe2x80x9cslurry reactorsxe2x80x9d or xe2x80x9cslurry bubble columns,xe2x80x9d operate by suspending catalytic particles in liquid and feeding gas reactants into the bottom of the reactor through a gas distributor, which produces small gas bubbles. As the gas bubbles rise through the reactor, the reactants are absorbed into the liquid and diffuse to the catalyst where, depending on the catalyst system, they are converted to gaseous and/or liquid products. The gaseous products enter the gas bubbles and are collected at the top of the reactor. Liquid products are recovered from the suspending liquid using different methods, for example, by passing the slurry through a filter that separates the liquid from the catalyst solids, and then separating the liquids.
A known problem in slurry reactors, however, is that water is continuously formed during Fisher-Tropsch synthesis in the reactors. This is known to limit conversion and prematurely deactivate catalyst systems such as iron and cobalt-based Fisher-Tropsch catalysts through an oxidation mechanism. As is well known in the art, a high water partial pressure correlates to a high deactivation rate. This is detrimental to the overall system performance, since two requirements for a successful commercial application of cobalt-based Fischer-Tropsch catalysts are a stable performance (low deactivation rate) and, for middle distillates production, a high wax selectivity (or a high alpha value).
For any given cobalt-based catalyst, along with the H2/CO ratio and the reaction temperature, the total pressure is a parameter that has a direct influence on the wax selectivity, in that a higher pressure will result in a higher wax selectivity. However, a higher total pressure (at any given degree of per-pass conversion) also correlates to a higher water partial pressure and therefore a higher deactivation rate. Therefore, if reactors are operated at conditions conducive to higher alpha values (higher pressures), per-pass conversion will necessarily have to be low to avoid premature catalyst deactivation due to water. A low per-pass conversion is undesirable, however, because it results in higher capital investment and operating costs. Additionally, for iron-based catalysts, the water not only has a negative effect on the catalyst deactivation rate, but it also inhibits the rate of reaction.
The water partial pressure is therefore a constraint that will not allow the realization of the kinetic and/or wax selectivity potential of iron and cobalt-based Fisher-Tropsch catalysts. Therefore, in order to improve the efficiency of slurry reactors using iron and cobalt-based catalyst systems, there exists a need for a method to remove water formed during Fisher-Tropsch synthesis.
The present invention relates to a system and method for water removal and optional liquid product separation in slurry reactor systems operating at Fischer-Tropsch conditions. More particularly, the present invention includes a water stripping system that allows the reaction water to be stripped and heavy liquid products to be removed in an external vessel. The term xe2x80x9cheavy liquidxe2x80x9d products is herein defined as hydrocarbons in the wax range, that is hydrocarbons heavier than Carbon 19. Generally, in stripping, a liquid containing a dissolved liquid or gas, such as water, flows down a column while a stripping gas flows up the column at conditions such that the dissolved liquid or gas comes out of solution and is carried off with the stripping gas. In the present invention this system can remove water dissolved in the wax, and potentially water contained in the very small bubbles in the wax, therefore allowing a higher per-pass conversion at pressures conducive to high alpha values while protecting the catalyst from excessive oxidation. By allowing a higher pass per conversion, fewer stages may be necessary to achieve a suitable overall conversion.
In a preferred embodiment of the present invention, a method for removing water from a slurry reactor containing a water-rich slurry includes removing a portion of water-rich slurry from the slurry reactor, stripping water from the water-rich slurry using an dry gas to form a water-reduced slurry and a water-rich gas stream, and returning the water-reduced slurry back to the reactor. Generally, the slurry includes gaseous reactants and products, liquid hydrocarbon products ranging from light liquid products to heavy liquid products, and catalyst. In some embodiments, a fraction of the heavy liquid products may be separated from the remaining slurry prior to returning the slurry back to the reactor.
In another preferred embodiment of the present invention, a method for producing hydrocarbons includes contacting a synthesis gas with a hydrocarbon synthesis catalyst in a slurry body comprising the catalyst and gas bubbles in a hydrocarbon slurry liquid having light and heavy components, under reaction conditions effective to form hydrocarbons and unavoidable secondary products, such as water, from the synthesis gas. A portion of the slurry from the slurry body then passes into a gas-disengaging zone to separate gas bubbles from the slurry and form gas-reduced slurry. Next, the gas-reduced slurry passes into a stripping zone, wherein the gas-reduced slurry is contacted with a dry stripping gas, which at least partially removes water therein to form water-reduced slurry. Lastly, the water reduced slurry passes back into the slurry body. In some embodiments, a fraction of the liquid component of the hydrocarbon slurry is separated from the remaining slurry prior to passing the slurry back into the slurry body.
The present invention allows higher per-pass conversions of syngas and/or use of higher total pressures at any given degree of conversion, while protecting the Fischer-Tropsch catalyst from an excessive oxidation rate. By returning the water-reduced slurry back into the reactor and optionally removing a fraction of the heavy liquid products, the catalyst inventory in the reactor is kept approximately constant. In some instances, circulation of the liquid back to the reactor may improve the temperature profile in the reactor.